Recently, the advancements of information technology (IT) have lead to the development of ubiquitous communication environments wherein various services can be provided between any types of apparatuses regardless of time and place. In addition, in a ubiquitous communication network, the use of wireless communication for connecting to different terminals has gradually increased due to advantages such as codeless operability and mobility.
In currently-used wireless communication technologies, RF/MW frequency bands vary from several MHz to tens of GHz, and a relatively low service rate is used in comparison with wired communication technologies. In addition, in wireless communication technologies, frequency sharing is needed among user terminals, satellites, and military networks. Also, problems such as information security and electromagnetic interference (EMI) harmful to human bodies need to be addressed.
As a promising technology for addressing the problems of the conventional wireless communication, there has been proposed an optical-wireless communication technology in which information is communicated using light propagating through a space.
According to current research results, an intensity modulation (IM)/direct detection (DD) optical-wireless communication technology can provide an inexpensive practical optical modulation scheme for indoor optical-wireless communication.
FIG. 1 is a conceptual view illustrating a conventional IM/DD optical-wireless communication apparatus.
Operations of the conventional optical-wireless communication apparatus are described in brief with reference to FIG. 1. An input electrical signal is converted into line codes suitable for optical channels by a modulator 10 and amplified into a suitable current signal by an amplifier. A light emitting device 20 such as a laser diode (LD) or a light emitting diode (LED) output an optical signal (optical channel) by using the current signal. The optical signal is detected by a photo diode (PD) 30 and converted into an optical current signal. The optical current signal is demodulated by a demodulator 40 and output to a receiver.
The modulation performed by the modulator 10 is classified into time-domain modulation and frequency-domain modulation. In the time-domain modulation, a non-return-to-zero (NRZ) or return-to-zero (RZ) code on-off keying scheme for modulating a transmission signal by using an on-off characteristic of a signal intensity and a pulse position modulation scheme for modulating the transmission signal by using a time difference between pulses are used. The frequency-domain modulation is performed by using one subcarrier or a plurality of subcarriers.
As a frequency-domain modulation scheme, orthogonal frequency division multiplexing (OFDM) is used. OFDM is widely used for wire/wireless communication such as x-digital subscriber line (xDSL), wireless local area network (LAN), and wireless Internet in which signal interference due to a multi-path has to be addressed. OFDM has advantages of easy implementation and easy frequency-band management.
In OFDM, an output signal has a large peek-to-average power ratio (PAPR). Therefore, an operating efficiency of a power amplifier at the output stage is lowered, and communication performance is deteriorated due to signal deformation caused by non-linearity of the power amplifier. Therefore, in IM optical-wireless communication using OFDM, due to a high PAPR, the power efficiency of the optical-wireless transmitter is lowered and nonlinear signal deformation occurs.
These problems are described in detail below with reference to FIG. 2.
FIG. 2 is a schematic block diagram illustrating a conventional IM optical-wireless transmitter.
Operations of the optical-wireless transmitter are described with reference to FIG. 2. An input electrical signal is modulated by a baseband modulator 201. The output signal of the baseband modulator 201 is a digital signal. The digital signal is converted into an analog signal by a digital-to-analog (DA) converter 202. The analog signal is amplified by a power amplifier 203. An LD or LED 204 is driven by the amplified signal to generate an optical-power signal in proportion to the output signal of the power amplifier 203.
As described above, in a case where the OFDM scheme is used for the baseband modulator 201, the output signal of the baseband modulator 201 has a high PAPR characteristic. Therefore, in order to linearly amplify the output signal, the power amplifier 203 needs to be implemented with class-A bias state. In general, a maximum power efficiency of the class-A amplifier is limited to 50% or less. The power efficiency of an amplifier is inversely proportional to the PAPR of the input signal. For example, if the PAPR is 10 dB, the power efficiency of the power amplifier 203 is about 5%.
A signal having a high PAPR characteristic undergoes signal deformation caused by the non-linearity of the power amplifier. In order to prevent the signal deformation, the power amplifier 203 need to have output power capacity larger by 10 dB˜20 dB than an average system power. According to our research results, an optical-wireless transmitter for 1 Gb/s˜3 Gb/s indoor optical-wireless communication needs to have an average optical power of 1 W. Such a high power optical-wireless transmitter is higher by 40 dB than typical 0.1 mW optical-wireless transmitter for ultra wide bandwidth (UWB) communication.
Therefore, if the 1 W optical-wireless transmitter is implemented by using conventional technologies, the power amplifier 203 needs to be implemented to have a maximum output power of 20 W to 40 W assuming 50% electrical-optical conversion efficiency of the LD/LED. In other words, a competitive 1 W optical-wireless transmitter cannot be implemented by using the conventional technologies.
This problem of high power consumption of the power amplifiers 203 has to be addressed for the competitiveness of the optical-wireless communication technology with respect to UWB technologies or 802.11n technologies. As a practical method, a large number of LDs or LEDs can be used to generate a 1 W optical signal while maintaining a high modulation rate (Gb/s). However, when the LDs such as vertical cavity surface emitting laser (VCSEL) diodes are operated with a high current, non-uniformity of the optical power intensities of the LDs occurs due to a difference of cooling structures of LD arrays. Therefore, an optical-wireless transmitter using a larger number of LDs needs to cope with the non-uniformity of the optical power intensities of the LDs.
In case of using a large number of LDs or LEDs, a larger number of drivers are also needed to drive the LDs or LEDs. In general, the drivers are made of different materials compared to the optical elements. Therefore, two devices separately formed must be connected to each other. However, for the connection between the two devices, a larger number of connection wires are needed. Thus, in order to implement an inexpensive optical-wireless transmitter, the number of connection wires between the drivers and the LDs or LEDs should be reduced.